Shock-absorbing devices are used in a wide variety of applications. For example, shock absorbing units have long been utilized in bicycles and motorcycles. Most modem bicycles and motorcycles embody some form of a shock-absorbing suspension system in front and rear fork assemblies and other body parts such as seat support structure for dampening the effects road irregularities impart to the rider. Another example where shock absorbing devices are commonly used involves draft gears at opposed ends of railroad cars. As well known, railroad car draft gears embody shock absorbing technology to lessen the effects impacting railroad cars have on one another.
Shock attenuating or absorbing devices typically embody a resilient member of a material having the property of self-restoration, namely restoring itself to the condition it had prior to its distortion by the application of an impact force or load thereto. One type of material having the property of self- restoration is an elastomer which has been widely used as both a tension and compression spring.
Present designs of mountain bicycles and off-road bicycles have telescopic front fork suspension systems which typically use a microcellular urethane material encapsulated within a cylindrical unit for absorbing energy or impacts as the bicycle traverses uneven or bumpy terrain. These designs, however, present problems because urethane type materials tend to absorb moisture thereby effecting their proper functioning, lack integrity and durability due to flaking, and have a spring rate that imposes the additional cost of a dampener upon the manufacturer of such suspension systems.
One of the more recent elastomers revealing promising results as a spring is a copolyester polymer. As disclosed in U.S. Pat. Nos. 4,198,037 and 4,566,678 to David G. Anderson and assigned to the assignee of the present application, thermoplastic elastomers have been widely used for the manufacture of compression springs. One such polymer is sold by DuPont & Co. under the tradename HYTREL.
Generally, this type of polymer elastomer material has inherent physical properties which make it unsuitable for use as a compression spring. As disclosed in the above-listed patents, however, applicant's assignee has advantageously discovered a method whereby copolyester polymer elastomer material can be processed to render the material usable for the manufacture of compression springs.
Thermoplastic polymer elastomer material has advantageously been used as a compression spring in railroad car draft gears. As will be appreciated by those skilled in the art, a railroad car draft gear typically includes a hollow housing or receptacle defining a blind cavity. The compression or shock-absorbing portion of the draft gear includes an elastomeric body including a series of axially spaced generally flat metals discs or plates axially inserted into the cavity. An end cap is arranged in operative combination with the compression spring and axially extends from the receptacle. As is well known, and during use on the railroad car, the end cap telescopically moves relative to the receptacle thereby compressing the shock-absorbing device therebetween by moving opposite ends of the shock-absorbing device toward each other resulting in the absorption of the energy or impact imparted thereto. Of course, after the impact or load is removed, the shock-absorbing device expands and automatically functions to return the end cap to a predetermined disposition relative to the receptacle.
Regardless of the particular application, elastomeric springs require a substantial space around it for the necessary lateral or radial expansion when the spring is compressed. Unfortunately, and as described above, such free space is not always available in practice. As will be appreciated by those skilled in the art, when the elastomeric spring is disposed in a tube or receptacle, radial expansion or bulging of the elastomeric spring, resulting from axial compression thereof, causes the periphery of the elastomeric spring to press against an inside diameter of the tube or receptacle which may result in stiction and an increased spring rate. Of course, stiction or an increased spring rate can easily and readily lead to twisting or snaking of the elastomeric spring within the tube or receptacle. Moreover, and as a result of axial compression under load, if any lengthwise portion of the elastomeric spring should radially expand or bulge outwardly beyond a predetermined limit, the tube or receptacle will tend to wear on the periphery of the elastomeric spring, thus, creating additional problems.
Thus, there is a need and a desire for an elastomeric shock attenuating apparatus or compression spring which is configured for use within tight space constraints as realized in a wide variety of applications such as bicycle suspension systems or railroad car draft gears and wherein the attenuating apparatus or spring eliminates the above-mentioned disadvantages while enhancing guidance and axial movements of the spring within a tube or receptacle on the bicycle suspension system or railroad car draft gear.